Gasification of coal in a rotary kiln is quite an old process as witnessed by U.S. Pat. No. 247,322, issued Sept. 30, 1881. The following is a list of patents which indicate generally the development of coal gasification in rotary kilns. It should be understood that this list of patents is not intended to be exhaustive but is merely a list of representative patents pertaining to coal gasification utilizing a rotary kiln: U.S. Pat. No. 596,428; French Pat. No. 149,049; U.S. Pat. No. 715,144; British 7,592; U.S. Pat. No. 868,026; U.S. Pat No. 1,159,675; U.S. Pat. No. 1,267,410; U.S. Pat. No. 1,270,949; U.S. Pat. No. 1,413,779; U.S. Pat. No. 1,423,134; U.S. Pat. No. 1,441,542; U.S. Pat. No. 1,480,148; U.S. Pat. No. 1,480,152; U.S. Pat. No. 2,659,668 and U.S. Pat. No. 3,692,505.
The recent realization of the growing "energy crisis" has pointed out the need for the United States to strive toward the goal of energy self-sufficiency by finding environmentally acceptable ways in which to utilize our greatest domestic energy resource - coal. It is quite universally accepted that coal gasification, followed subsequently by appropriate gas cleanup and sulfur removal, will play a major role in the successful utilization of the coal deposits in the United States.
One use of gasified coal is the production of electricity. In such a process, a low BTU gas produced from coal is used as a utility steam boiler or combined cycle fuel. In order for such process to be viable and economically attractive for utility application, two inherent prerequisites must be fulfilled: (1) tremendously large amounts of coal must be gasified, and (2) the coal most readily available to the utility site should be utilizable. It is interesting to note that most of the electrical demand and hence the most electrical utility generating sites within the U.S. are located east of the Mississippi. Unfortunately, most of the known coal reserves in the same geographic region are typically of a moderate to strongly caking variety which historically have been difficult to process.
In summary, this creates a demand for a gasification process capable of processing quantities of coal on an unprecendented scale (100's of tons per hour) with coals exhibiting a wide range of caking properties not readily processed by currently available commercial process technology. One of the more efficient ways of gasifying large amounts of caking coal is in a continuous gasifying process utilizing a rotary kiln.
Coal is a complex substance and therefore does present quite a few problems which must be solved before successful economical gasification can be realized. Not the least of these problems is the caking characteristics which many of these coals exhibit. In order to gasify the coal, the temperature of the coal has to be increased to high temperatures. In order to achieve practical rates of gasification, this temperature typically ranges between 1600.degree. - 2600.degree. F. As the coal temperature is increased, the coal passes through a semi-plastic state in which it becomes sticky due to the evolution of volatile and tar products and other factors related to continuous changes in the coal structure. While in this state, (which can be referred to as the agglomerating temperature range) pieces of coal stick together and form agglomerates which, in certain instances, can adversely affect the proper operation of the coal gasifier. In some instances, the agglomerates may reach sufficient size and number so as to upset normal continuous material flow through the process. In other instances, agglomerates may actually clog the kiln discharge opening or other process equipment downstream from the kiln.
In the past, efforts were made to entirely avoid caking coal from forming agglomerates. Examples of such efforts can be found in the following patents: U.S. Pat. No. 1,159,675, issued to J. W. Hornsey; U.S. Pat. No. 1,916,900, issued to J. N. Vandegrift et al; U.S. Pat. No. 2,659,668, issued to B. J. Mayland, Nov. 17, 1953; U.S. Pat. No. 3,692,505, issued to E. H. Reichl, Sept. 19, 1972 and British Pat. No. 278,378. The Hornsey patent attempts to prohibit agglomeration by using coal in powder form and providing buckets, cradles and rings in the kiln which sufficiently agitate the coal so that agglomerates do not form. Both the Vandegrift and Mayland patents attempt to prohibit agglomeration by treating coal with gas or air to provide a nonsticky layer about the coal so that as it passes through the semi-plastic state, the surface is not sticky and therefore does not adhere to adjacent coal particles. The Reichl patent attempts to prohibit agglomeration by introducing noncaking pellets into the kiln in sufficient proportions to ensure that the resultant mixed bed will not cake. The British patent indicates that the caking coal must be mixed with other substances, such as water, to reduce the caking tendency.